This invention relates generally to fluid velocity measurement and imaging, and more particularly the invention relates to improved velocity measurement of an incompressible fluid using divergence-free constraint of true velocity.
Several technologies including Doppler ultrasound and magnetic resonance (MR) are known for measuring and imaging fluid flow. In Doppler ultrasound, the phase of reflected ultrasonic waves due to the flow of fluid provides a measure of velocity. In magnetic resonance imaging, bipolar flow encoding gradients can be used in a three-dimensional MRI procedure to provide a measure of flow. Pelc et al. "Encoding Strategies for Three Direction Phase Contrast MR Imaging of Flow" Journal of Magnetic Resonance Imaging, Vol. I, No. 4, pp. 405-413, 1991 discloses encoding strategies for the measurement of flow velocities in arbitrary directions with phase-contrast magnetic resonance imaging.
Phase sensitive MR methods of flow measurement have certain limitations. For example, velocity induced phase shift is proportional to velocity, and dispersions in velocity result in dispersions of phase shift which, in turn, can result in attenuation of observable signal when the flow is complex. Further, imaging of blood flow in blood vessels necessitates a high signal to noise ratio to provide adequate image resolution. As described by Dumoulin et al. "Three Dimensional Phase Contrast Angiography" Magnetic Resonance in Medicine 9, pp. 139-149 (1989), three-dimensional Fourier transform (3DFT) imaging can be used to avoid some of the problems of phase sensitive flow techniques.
The 3DFT imaging provides a high signal noise ratio for a given time span because each acquired echo contains signals from all image voxels. Further, the volume imaging can unambiguously identify the location of a small vessel in surrounding anatomy. Thus, MR angiograms can be produced by computing the speed at each voxel and reprojecting the computed speed at one or more viewing angles.
The present invention is directed to reducing noise and increasing conspicuity of phase contrast velocity measurements and images.